1. Field of the Invention
The present invention relates to an image forming apparatus, and in particular relates to art for controlling a dot diameter on a recording medium.
2. Description of the Related Art
Inkjet recording apparatuses (inkjet printers) according to which an image is formed by dots on a recording medium formed by liquid ejected from a plurality of nozzles formed in heads (liquid ejection heads) while the heads and the recording medium are moved relatively to one another, have been known.
With such an inkjet recording apparatus, by making the recording density (the number of dots that can be deposited per unit area) be variable, there can be provided a system according to which both high quality and high speed can be achieved. For example, when high quality is considered important, the recording density is made high, whereas when high speed is considered important, the recording density is made low. However, in the case where the recording density is made low, if dots of the same size as that when the recording density is high are deposited, then there is little overlap of dots, and in some cases, voids may arise between dots. A method in which the dot diameter (diameter) φ is changed by changing the droplet volume V of the droplets ejected from the nozzles has thus been known. It is well known that the relationship between φ and V is in general “φ ∝ V1/3”. However, with a system in which the droplet volume V is made to be variable in this way, it is necessary to eject droplets of different droplet volume V depending on the recording density, and hence a plurality of different waveforms for the piezoelectric elements in the heads are required to be prepared.
A method in which the dot diameter is controlled with a single waveform (i.e. a single droplet volume V) has thus been proposed in, for example, Japanese Patent Application Publication No. 2004-9483. According to Japanese Patent Application Publication No. 2004-9483, in a system in which an ultraviolet radiation (UV)-curable ink is used, the dot diameter is controlled depending on the way of applying UV (the time interval from droplet deposition to irradiation, the duration of the irradiation, and the intensity of the irradiation) being changed in accordance with the recording medium or the recording mode.
However, in the case of a system in which an ultraviolet radiation-curable ink is directly deposited onto a recording medium as in Japanese Patent Application Publication No. 2004-9483, each droplet spreads out up to a certain time T0 (in general, a few μsec to a few tens of μsec) after landing, and then reaches equilibrium. That is, if the time interval from the deposition of a droplet to the UV irradiation is less than T0, then the dot diameter can be controlled through the way of applying the UV; however, if the time is beyond the time interval T0, then it is difficult to control the dot diameter since the droplet has reached equilibrium. Here, defining the “spreading ratio” as the ratio of the diameter of a dot that has landed on the recording medium to the diameter assuming the droplet to be spherical, the spreading ratio of the dot which has been once reached equilibrium as above is approximately 2. Hence, with the art disclosed in Japanese Patent Application Publication No. 2004-9483, the control can be carried out only with respect to a dot having a spreading ratio of up to approximately 2, and it is difficult to control the dot diameter over a wide range.